The present invention relates to a coating composition for preparing a back coating film of a mirror. The back coating film prepared by curing the coating composition provides the mirror with anticorrosion property.
Conventional back coating films for mirror contain corrosion-inhibitive lead pigments, such as red lead, white red, basic lead sulfate, lead cyanamide, and calcium plumbate. The incorporation of such lead pigment has so-called advantageous cation effect, reducing effect, neutralization effect, and anion effect. By this cation effect, ionization and deterioration of metal(s) (i.e., silver, copper and/or at least one other metal) of mirror are suppressed by lead having an ionization tendency greater than those of these metals. By the reducing effect, a reducing atmosphere is produced at the interface between the back coating film and the silver mirror film or the metallic (e.g., copper) protective film, thereby to suppress oxidation of the silver mirror film and the metallic protective film. By the a neutralization effect, a rust of the silver film and/or of the metallic protective film is neutralized by a basic substance generated from a lead pigment, thereby to suppress oxidation of the silver mirror film and/or the metallic protective film. By the anion effect, an anion (e.g., that of cyanamide) reacts with a metal ion derived from the silver mirror film and/or from the metallic protective film. With this, there is formed a passive film that suppresses corrosion of the silver mirror film and the metallic protective film. By the above-mentioned cation, reducing, neutralization, anion effects, the occurrence of corrosion and deterioration of the peripheral and inner portions of the mirror can be prevented.
Japanese Patent Unexamined Publication JP-A-7-261007 discloses a coating composition for preparing a back coating film of a mirror. This coating composition contains a binder of a bisphenol-type epoxy ester resin, benzotriazole and/or its derivative, red lead, and the like (see Tables 1-2). The use of lead in mirror, however, is not desirable from an environmental point of view, as will be mentioned hereinafter. Nevertheless, if the lead pigment is omitted in this coating composition, its anticorrosion property becomes inferior to those containing a lead pigment.
JP-A-7-234306 discloses a mirror that has a special film 4 interposed between a copper protective film 3 and a back coating film 5, in order to improve the mirror in chemical durability. This special film 4 is prepared by applying benzotriazole and/or its derivative to the copper protective film 3. In this publication, it is stated that the back coating film 5 may be formed by using one of various conventional alkyd resin paints, epoxy resin paints, and epoxy ester resin paints. It is known that these paints contain a lead pigment(s). As stated above, the use of lead in mirror is not desirable. In spite of that, if the lead pigment is omitted in the back coating film, the mirror may become unsatisfactory in anticorrosion property.
The use of lead in mirror may cause an environmental hazard. According to an analysis conducted by an examination organization (TCLP) certified by U.S. Environmental Protection Agency (EPA), it was found that conventional mirrors having a back coating film containing a lead pigment contain 30-60 mg/L of lead. Under the EPA""s regulation, this concentration is far greater than the permissible upper limit of the lead concentration (i.e., 5.0 mg/L) of wastes to be dumped. Thus, in the United States, it is prohibited to dump cullet and wastes of such mirrors. In view of this, there is a strong demand for a mirror that is substantially free of lead and has advantageous characteristics, in mechanical processability and chemical and mechanical durabilities, which are comparable to those of a mirror having a back coating film containing a lead pigment.
It is therefore an object of the present invention to provide a coating composition for preparing a back coating film of a mirror that is superior in anticorrosion property and is substantially free of lead to reduce a risk of the environmental impact.
It is another object of the present invention to provide such anticorrosive mirror having the back coating film prepared by curing the coating composition.
It is a more specific object of the present invention to provide an anticorrosive mirror that is substantially free of lead and has advantageous characteristics, in mechanical processability (e.g., in cutting and chamfering) and chemical and mechanical durabilities, which are comparable to those of a mirror having a back coating film containing a lead pigment.
According to the present invention, there is provided a coating composition for preparing a back coating film of a mirror. This mirror serving as an anticorrosive mirror has a glass substrate, a silver mirror film that comprises silver and is formed on a back surface of the glass substrate, a metallic protective film that comprises a first metal and is formed on the silver mirror film, and the back coating film formed on the metallic protective film. The coating composition is substantially free of a lead component and comprises (a) a vehicle containing a synthetic resin, (b) a first salt formed by a neutralization of an acid with a base containing a second metal having an ionization tendency that is greater than the silver and the first metal, and (c) a basic compound selected from the group consisting of azoles and diamines. The acid is one compound selected from the group consisting of organic acids each containing an azole ring and unsubstituted and substituted carbamic acids.
According to the present invention, there is further provided such an anticorrosive mirror comprising (a) a glass substrate, (b) a silver mirror film that comprises silver and is formed on a back surface of the glass substrate, (c) a metallic protective film that comprises a first metal and is formed on the silver mirror film, and (d) a back coating film that is formed on the metallic protective film and is prepared by curing the above-mentioned coating composition.
An anticorrosive mirror according to the present invention can be formed at first by forming on a glass substrate a silver mirror film and then a metallic protective film, for example, by spraying, and then by forming on the metallic protective film a back coating film for providing the mirror with anticorrosion property. The silver mirror film contains silver and may further include at least one other metal such as chromium and titanium. The metallic protective film contains a first metal which is not particularly limited. Copper, which is the most commonly used for conventional metallic protective films, may be used as the first metal. The back coating film is formed at first by applying a coating composition according to the present invention to the metallic protective film and then by curing the coating composition. This application may be conducted, for example, by roller coating, flow coating, and spraying.
As stated above, a coating composition according to the present invention for preparing the back coating film is substantially free of a lead component and comprises (a) a vehicle containing a synthetic resin, (b) a first salt formed by a neutralization of an acid with a base containing a second metal (e.g., zinc or iron) having an ionization tendency that is greater than the silver and the first metal (e.g., copper), and (c) a basic compound selected from the group consisting of azoles and diamines. The acid is one compound selected from the group consisting of organic acids each containing an azole ring and unsubstituted and substituted carbamic acids. Each of the first salt and the basic compound can serve as an anticorrosive agent for the mirror. When both of the first salt and the basic compound are used together, there may be obtained the above-mentioned advantageous cation, reducing, neutralization and anion effects, which are comparable to those of a lead pigment.
The first salt of the coating composition has the following advantageous effects. As stated above, the second metal has an ionization tendency that is greater than the silver and the first metal (e.g., copper). Therefore, ionization of the silver and the first metal is effectively suppressed by the second metal. Even if the silver and/or the first metal is ionized to produce a cation, an anion obtained by dissociation of the first salt may react with this cation, thereby to form a passive film. This passive film may increase adhesion of the back coating film to the metallic protective film and/or to the silver mirror film, and suppress the infiltration of corrosive substances. The first salt can be at least one metal salt selected from the group consisting of metal thiazoles, metal benzimidazoles and metal dithiocarbamates. Nonlimitative examples of the first salt are zinc 2-mercaptobenzothiazole, zinc dimethyldithiocarbamate, zinc diethyldithiocarbamate, zinc dibutyldithiocarbamate, zinc N-ethyl-N-phenyldithiocarbamate, zinc N-pentamethylenedithiocarbamate, zinc dibenzyldithiocarbamate, ferric dimethyldithiocarbamate, zinc 2-mercaptobenzimidazole, and zinc 2-mercaptomethylbenzimidazole. The first salt, for example, zinc 2-mercaptobenzothiazole, may be dissociated, as shown in the following reaction formula (1), and may react with the first metal (e.g., copper) and/or silver, thereby to form copper 2-mercaptobenzothiazole and/or silver 2-mercaptobenzothiazole, as shown in the following reaction formulas (2) and (3). With this, the infiltration of corrosive substances may be suppressed, as stated above. 
In the invention, the first salt can be in an amount of from 1 to 15 wt %, furthermore from 3 to 11 wt %, of the solid matter contained in the coating composition. If it is less than 1 wt %, adhesion of the back coating film to the metallic protective film may become inferior. Furthermore, the mirror may become unsatisfactory in the after-mentioned salt spray test, copper accelerated acetic acid salt spray (CASS) test, and other tests. If it is greater than 15 wt %, viscosity of the coating composition may become too high during its storage, and its pigment tends to precipitate therein.
Conventional back coating films for mirror do not serve as a high barrier to suppress the permeation of gas molecules therethrough. Similarly, the silver mirror film and the metallic protective film for mirror in accordance with conventional technologies and the present invention allow the permeation of gas molecules therethrough, because these films are each prepared by the accumulation of the respective metal colloid particles. Thus, according to conventional mirrors, corrosive substances (e.g., oxygen and hydrogen) in the form of gas may permeate through the back coating film, then the metallic protective film, and then the silver mirror film, and thus may reach the surface of the glass substrate. In contrast, according to the present invention, the basic compound, which is selected from the group consisting of azoles and diamines and derived from the back coating film, tends to react with silver ions, copper ions, and/or other metal ions. With this, a rust of the silver mirror film and/or the metallic protective film may be neutralized, and thus the growth of rust of these films may be suppressed. The basic compound of the present invention can be at least one compound selected from the group consisting of imidazole derivatives, thiazole derivatives, and phenylenediamine derivatives. Nonlimitative examples of the basic compound are 2-methylimidazole, 2-ethyl-4-methylimidazole, 2-undecylimidazole, 2-heptadecylimidazole, 2-phenylimidazole, 2-phenyl-4-methylimidazole, 1-benzyl-2-methylimidazole, 1-cyanoethyl-2-methylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole, 1-cyanoethyl-2-undecylimidazole, 1-cyanoethyl-2-phenylimidazole, 1-cyanoethyl-2-methylimidazolium trimellitate, 1-cyanoethyl-2-ethyl-4-methylimidazolium trimellitate, 1-cyanoethyl-2-undecylimidazolium trimellitate, 1-cyanoethyl-2-phenylimidazolium trimellitate, 2,4-diamino-6-[2xe2x80x2-methylimidazolyl-(1xe2x80x2)]ethyl-S-triazine, 2,4-diamino-6-[2xe2x80x2-ethyl-4xe2x80x2-methylimidazolyl-(1xe2x80x2)]ethyl-S-triazine, 2,4-diamino-6-[2xe2x80x2-undecylimidazolyl-(1xe2x80x2)]ethyl-S-triazine, adducts of 2,4-diamino-6-[2xe2x80x2-methylimidazolyl-(1xe2x80x2)]ethyl-S-triazine and isocyanuric acid, adducts of 2-methylimidazole and isocyanuric acid, adducts of 2-phenylimidazole and isocyanuric acid, 2-phenyl-4,5-dihydroxymethylimidazole, 2-phenyl-4-methyl-5-hydroxymethylimidazole, 1-cyanoethyl-2-phenyl-4,5-dicyanoethoxymethylimidazole, 2-phenylimidazoline, 2-mercaptobenzimidazole, 2-mercaptomethylbenzimidazole, 2-mercaptobenzthiazole cyclohexylamine, 2-mercaptobenzthiazole dicyclohexylamine, m-phenylenediamine, p-phenylenediamine, p-(p-toluenesulfonylamide)diphenylamine, N,Nxe2x80x2-di-2-naphthyl-p-phenylenediamine, N,Nxe2x80x2-diphenyl-p-phenylenediamine, 3,5-diaminochlorobenzene, 3,5-phenylenediamine oxide, 2,4-diaminotoluene, 2,4-diaminophenol, 2,4-diaminodiphenylamine, 3,4-diaminobenzophenone, 2,4,6-trimethyl-1,3-phenylenediamine, 2,5-dimethyl-p-phenylenediamine, N,Nxe2x80x2-dimethyl-p-phenylenediamine, N-methyl-Nxe2x80x2-ethyl-p-phenylenediamine, N-methyl-Nxe2x80x2-isopropyl-p-phenylenediamine, N,Nxe2x80x2-diisopropyl-p-phenylenediamine, N-phenyl-Nxe2x80x2-isopropyl-p-phenylenediamine, and N-phenyl-Nxe2x80x2-(1,3-dimethylbutyl)-p-phenylenediamine. For example, 2,4-diamino-6-[2xe2x80x2-methylimidazolyl-(1xe2x80x2)]ethyl-S-triazine, which is a basic compound of the present invention, may be decomposed by heat, as shown in the following reaction formula (4). The thus produced 2-methylimidazole may react with silver, copper, and/or at least one other metal, as shown in the following reaction formulas (5) and (6). With this, a rust of the silver mirror film and/or the metallic protective film may be neutralized, and thus the growth of rust of these films may be suppressed. 
In the invention, the basic compound can be in an amount of from 0.5 to 5 wt %, furthermore from 1.5 to 3.5 wt %, of the solid matter contained in the coating composition. If it is less than 0.5 wt %, the mirror may become unsatisfactory in the after-mentioned salt spray test, CASS test, and other tests. If it is greater than 5 wt %, the back coating film may have blisters in the after-mentioned water resistance test.
In the invention, a synthetic-resin vehicle for preparing the back coating film is not particularly limited, and it can be selected from thermoplastic and thermosetting resins, depending on the condition for applying the back coating film and on the degree of durability that is required for mirror. Nonlimitative examples of the synthetic-resin vehicle are acrylic resins, polyester resins, alkyd resins, various modified alkyd resins, epoxy resins, urethane resins, fluororesins, vinyl resins, nitrocellulose-modified resins, phenol resins, amino-modified resins, and cellulose acetate butylate modified resins.
In the invention, the coating composition may contain at least one pigment. In the preparation of the coating composition, the weight ratio of the total of the at least one pigment, the first salt and the basic compound to the synthetic-resin in the form of solid can be in a range of from 1:1 to 3:1, furthermore from 1.4:1 to 2.2:1 If it is less than 1:1, the back coating film may become insufficient in hardness. Thus, it tends to have scratches thereon and may have blisters in the water resistance test. If it is greater than 3:1, adhesion of the back coating film to the metallic protective film may become inferior. Thus, the mirror may become inferior in chemical resistance, weather resistance, durability, and the like, and tends to have corrosion and/or deterioration at its peripheral portion and/or its inner portion.
In the invention, nonlimitative examples of the first metal other than copper are nickel, nickel alloys, tin, and tin alloys, and nonlimitative examples of the second metal are Zn, Fe, and Te.